SPST vs SPDT vs DPST vs DPDT Relays: Which Contact Type Do You Actually Need?
One of my guys once pulled a DPDT relay out of the spares bin for a job that only needed an SPST. Cost us a spare part, and twenty minutes retracing wires that never needed to exist. That mix-up is exactly why SPST vs SPDT vs DPST vs DPDT relays trips up so many people on the plant floor, even good techs. The names sound like alphabet soup, but they only describe two things: how many circuits the relay switches, and how many positions each circuit can land on.
I’ve wired these into everything from pump starters to three-phase motor panels over 15 years in industrial automation, and the mix-up is almost always the same one. Someone counts terminals instead of checking the actual contact diagram, or grabs whatever relay happens to already be in the drawer. This guide walks through what each type does, where it fits, and where I’ve seen people get burned picking the wrong one. By the end, you’ll know exactly which contact type your circuit needs, no more guessing.
Table of Contents
SPST vs SPDT vs DPST vs DPDT
SPST, SPDT, DPST, and DPDT relays are all defined by two numbers: poles and throws. SPST switches one circuit on or off. SPDT switches one circuit between two paths. DPST switches two circuits on or off together. DPDT switches two circuits between two paths at the same time. Pick based on how many circuits you need to control, and whether each one needs a simple on/off or a full changeover.
| Type | Poles | Throws | Terminals | What It Does |
|---|---|---|---|---|
| SPST | 1 | 1 | 2 | Turns one circuit on or off |
| SPDT | 1 | 2 | 3 | Switches one circuit between two outputs |
| DPST | 2 | 1 | 4 | Turns two circuits on or off together |
| DPDT | 2 | 2 | 6 | Switches two circuits between two outputs together |
If you remember nothing else from this article, remember this: pole counts circuits, throw counts positions. That one line untangles every acronym on this list.
What Pole and Throw Actually Mean on a Relay
A pole is one independent circuit path controlled by the relay’s coil. A throw is the number of output positions that circuit can connect to. Say those two words out loud before you look at a datasheet, and the rest of this gets simple fast.
Single pole means one circuit. Double pole means two circuits, wired separately but switched at the same instant by the same coil. Single throw means the circuit only closes, like a basic light switch. Double throw means the circuit moves between two positions, normally open and normally closed.
Here’s the part that trips people up. SPDT and DPST both show up with four terminals in some catalogs, and they look close to identical at a glance. They are not the same thing at all. SPDT is one circuit that can go two places. DPST is two separate circuits that only go one place each, together. In my experience, this exact mix-up causes more miswiring calls than any other confusion on this list. Mix them up, and a changeover circuit ends up wired like a plain on/off, and nothing behaves the way the ladder diagram says it should.
SPST Relay: One Circuit, One Path
An SPST relay controls one circuit with a simple on or off action, nothing more. It has two terminals, common and normally open (or normally closed, depending on the part), and that’s the whole story.
I reach for SPST on the simplest jobs: a single indicator lamp, a basic interlock, an alarm that just needs to fire or not fire. If the job never needs a second position or a second circuit, SPST does it with the fewest parts and the least wiring to get wrong.
One thing to watch: some catalogs list SPST-NO and SPST-NC as separate part numbers. Order the wrong one, and your “normally open” contact shows up normally closed, energized backward from what your ladder diagram expects.
SPDT Relay: The Changeover Contact
An SPDT relay switches one circuit between two output paths, common to either normally open or normally closed. Techs usually just call it a changeover contact, and that name tells you exactly what it does.
This is the relay you grab for a manual/auto selector, an alarm that needs a normal state and a fault state, or any spot where one signal has to flip between two conditions. It’s also the most stocked relay type in most parts rooms, because one part gives you both wiring options instead of forcing you to buy two different SPST parts.
If you only remember one relay type from this whole guide, make it SPDT. It covers more real jobs than the other three combined.
Need help picking the right relay for your panel? Talk to your supplier’s application engineer before you order. A five-minute call beats a wrong part number sitting in a drawer for two years.
DPST Relay: Two Circuits, One Action
A DPST relay switches two independent circuits on or off at the same time, using one coil. Think of it as two SPST relays bolted together and fired by the same signal.
The classic use is switching line and neutral together, or running two isolated control circuits off one trigger without letting them touch electrically. It also shows up in some redundant signaling setups, sending the same on/off state to two separate destinations.
But here’s the thing people get wrong: DPST does not automatically mean fail-safe or redundant. Two poles switching together is not the same as two independent safety paths. If your job actually needs redundancy, the contact rating, wiring method, and applicable safety standard still have to check out on their own. Don’t let the acronym do that thinking for you.
DPDT Relay: Two Changeover Contacts in One Body
A DPDT relay is two SPDT contact sets operated together by one coil, giving you two circuits that each switch between two paths. Six terminals total: two commons, two normally-open, two normally-closed.
This is your motor reversing relay, your polarity flip circuit, and your go-to when you need two changeover functions to happen at the same instant. It’s also the easiest of the four to miswire, since six terminals give you more ways to cross a connection than two or three terminals ever will.
And it gets better once you see the pattern: if SPDT fits your job and you need two of them acting together, DPDT is just that, nothing more exotic underneath.
Getting the SPST vs SPDT vs DPST vs DPDT relay choice right before you order avoids most of what’s coming up in the mistakes section below.
Coil Voltage vs Contact Configuration: Two Separate Specs
A relay’s coil voltage and its contact configuration are two completely separate specs, and mixing them up is a common ordering mistake. The coil side energizes the relay. The contact side is what the relay actually switches. A 24V DC coil relay can carry SPST, SPDT, DPST, or DPDT contacts. Same goes for a 120V or 230V coil.
I’ve seen techs order a relay purely by coil voltage and assume the contact type would just match the old part. It doesn’t work that way. Check both specs every time: coil voltage for the control side, pole and throw for the load side, before you write the part number down.
Contact Ratings and Load Type: Why AC-15 and DC-13 Matter
Pole and throw tell you the circuit logic. They tell you nothing about whether the relay can safely switch your actual load. That’s what contact ratings are for, and skipping this step is where a lot of relay failures start.
Utilization categories such as AC-15 (AC control of electromagnetic loads) and DC-13 (DC control of electromagnets) set a lower current rating than the same relay’s plain resistive rating. Coils, contactors, solenoid valves, and small motors all pull an inrush current well above their running current. A relay rated for 10A resistive might only handle a fraction of that on an inductive load like a contactor coil.
Some engineers argue you can always oversize the relay and skip the derating math to save time. That works right up until the contacts weld shut from repeated inrush, and then you’re troubleshooting a relay that looks fine on the outside but never opens the circuit again. Check the load type against the rated category first. It takes two minutes and saves a callback at 2 a.m.
For a deeper look at how relays and contact classifications actually work under the hood,
Which One to Use: Motor Control, Valves, Signaling, and Redundancy
Match the relay to the job by asking two questions: how many circuits, and does each one need on/off or changeover. A single-phase load with no safety requirement usually needs nothing more than SPST or SPDT. A three-phase motor is a different story. US panel designs following NEC guidance generally call for breaking more than one line conductor for motor disconnects, which is why DPST or DPDT, or a contactor built the same way, is the minimum, not SPST.
Valves and simple alarms run fine on SPST or SPDT in most cases. Panel signaling that needs both a normal state and a fault state almost always wants SPDT, since one contact gives you both. Redundant signaling, where the same state needs to reach two separate destinations, points toward DPST. Motor direction control, and any job needing two changeover functions at once, needs DPDT.
Quick gut check: if you’re reaching for DPDT out of habit because “it does more,” stop and confirm the job actually needs two changeover circuits. Extra poles mean extra terminals, extra wiring, and extra chances to cross a wire during a rushed startup.
Common Wiring Mistakes That Cost You a Callback
These are the mistakes I see most on site, roughly in order of how much trouble they cause.
Counting terminals instead of reading the diagram tops the list. Some relays carry extra terminals for indicator lamps or internal jumpers, and terminal count alone can fool you into misreading the pole count.
Treating DPDT as a universal motor reversing fix comes next. The contact logic can flip polarity, but the motor still needs the right overload protection, mechanical or electrical interlocking, and a contact rating that matches the starting current. A bare DPDT relay with no protection is not a reversing starter.
Assuming DPST equals safety redundancy is another one. Two poles switching together share the same coil and the same failure point. Real redundancy needs independent paths, checked against the standard that actually applies to your safety circuit.
Last one: mixing up coil voltage with contact rating when reordering a spare. Write down both numbers before you call the supplier, not just the one printed biggest on the old relay’s label.
FAQ
What is the difference between SPST and SPDT relay?
SPST turns one circuit on or off. SPDT switches that same single circuit between two output paths instead of just on or off.
What is the difference between DPST and DPDT?
DPST turns two circuits on or off together. DPDT switches those two circuits between two paths each, at the same time.
Is SPDT the same as a changeover contact?
Yes. In relay and panel work, SPDT and changeover contact mean the same thing.
Can a DPDT relay reverse a motor by itself?
The contact logic can flip polarity, but you still need proper overload protection and interlocking. Never use a bare DPDT relay as your only reversing safeguard.
Which relay do I need for a three-phase motor?
DPST or DPDT at minimum, since single-pole relays like SPST don’t break enough conductors for a three-phase disconnect.
How do I check what contact type my relay actually has?
Read the wiring diagram or the terminal markings on the relay body, not just the terminal count.
Is a DPDT relay just two SPDT relays?
Functionally, yes, two SPDT contact sets sharing one coil. Electrically, the two poles stay isolated from each other even though they switch together.
Conclusion
SPST vs SPDT vs DPST vs DPDT relays comes down to two questions every time: how many circuits, and on/off or changeover. Get those two answers before you open a catalog, and you’ll stop paying for confusion with wasted parts and callbacks. Next time you’re staring at a bin of relays, count poles first, throws second, and check the load rating before anything gets wired in.
Still not sure which relay fits your panel? Pull up your wiring diagram, check the load type against the contact rating chart in this guide, and you’ll have your answer in under five minutes.
